Mechanisms for detecting neutrons in matter are generally based on indirect methods. Neutrons are generally detected by the signatures they produce through interactions with surrounding material. Such interactions include elastic scattering producing a recoiling nucleus, inelastic scattering producing an excited nucleus, or absorption with transmutation of the resulting nucleus. Most detection approaches rely on detecting the various reaction products of such interactions.
In one type of neutron interaction with matter, high-energy neutrons are scattered by a nucleus, transferring some of the kinetic energy of the neutrons to the nucleus. If enough energy is transferred, the recoiling nucleus ionizes the material surrounding the point of interaction. Since the maximum transfer of energy occurs when the mass of the atom with which the neutron collides is comparable to the neutron mass, hydrogenous materials are often the preferred medium for such detectors. In another type of neutron interaction with matter, low-energy (“slow”) neutrons react with surrounding absorber materials to produce absorption products, such as protons, alpha particles, gamma rays, and fission fragments. Typical absorber materials used in this type of detection have high cross sections for absorption of neutrons, and include Helium-3 (3He), Lithium-6 (6Li), Boron-10 (10B), and Uranium-235 (235U). Each of these reacts with neutrons to produce high-energy ionized particles that can be detected by different means.
Detectors employing either target nuclei or nuclear reactions use solid, liquid, or gas-filled detection media. A majority of neutron detectors in use today are gas-filled proportional counters, and in particular, either 10BF3 or 3He gas proportional tubes.
Because slow neutrons have insufficient energy to ionize materials directly, a nucleus with high neutron absorption cross-section is added to gas-filled detectors to facilitate detection. Nuclei commonly used for this purpose are 10B and 3He. In gas-filled proportional neutron detectors using 3He as the fill gas, the neutron reacts with the 3He nucleus resulting in the production of a triton (the nucleus of tritium, 3H) and a proton. The triton and the proton share the reaction energy of 765-keV (kilo-electron volts). These energetic particles generate electrons by ionizing collisions with fill-gas atoms. The electrons are accelerated by a high voltage (1300 to 2000 volts) maintained in the proportional counter, and this results in an electrical discharge that is detected as an electrical signal. In gas-filled proportional neutron detectors using BF3 as the fill gas, absorption of a neutron by 10B results in the production of 4He and 7Li, with 2310 keV shared between them. The 7Li is left in an excited state with 93% probability from which it subsequently decays by emitting a 480-keV gamma ray. The energetic products of the neutron reaction generate an electrical discharge in the fill gas by a mechanism similar to that of the 3He proportional counter.
Many instruments in the field use BF3, but because BF3 is toxic and corrosive, the use of 3He has traditionally been preferred. 3He proportional tube detectors have higher efficiencies, with none of the disadvantages of BF3. All proportional detectors require high voltages to produce electrical discharges, are susceptible to microphonic noise, and have a dead time of approximately 1 microsecond that limits their maximum counting rate. The tubes also require an ultra-pure quench gas (usually CO2) to achieve a sufficient signal-to-noise ratio, and suffer from wall effects when particle energy is lost by absorption at the tube walls.
Despite the above disadvantages, 3He proportional tube detectors are effective and are the preferred choice in many types of operations, including oil well logging and medical applications such as diagnosis of chronic obstructive pulmonary diseases. The supply of 3He is limited, and therefore, large-scale deployment of 3He is not currently possible. Alternatives to 3He-based neutron detection are necessary to meet the needs for highly sensitive neutron detectors having neutron/gamma discrimination similar to those of 3He detectors. Such detectors are required for safeguarding nuclear materials and weapons, treaty verification, anti-proliferation, recovery of lost military payloads, surveillance at border and port facilities, transportation systems and other places through which large amounts of material pass on a regular basis.
Another class of conventional neutron detectors is scintillation-based detectors. Such detectors are based on photon emission resulting from the interaction of energetic charged nuclei released from collisions between incident neutrons and atomic nuclei with scintillation materials. Scintillation devices are typically coupled to a photon detector that generates an analog electrical signal based on the production of the light within the scintillation material. The photon detector analog signal is a measure of the incident neutron irradiation. To enhance the efficiencies of the scintillators, neutron sensitive materials are typically doped with 6Li and 10B. However, neutron/gamma ray discrimination remains an issue for scintillators, and must be resolved in order for scintillators to becoming practical for 3He replacement.
Another class of neutron detectors includes solid state neutron detection devices based on thin films of 10B or 6Li coated onto silicon and other substrates. Losses in the substrate limit the ultimate efficiency of multi-layer detectors of this type.
A need exists for highly sensitive neutron detectors having neutron/gamma discrimination similar to 3He detectors.